First off, you've got your titles b0rked.. Jedi Academy was the sequel to Jedi Outcast. Jedi Outcast IS on the Gamecube (Jedi Academy is only on XBox and PC), but had a short run.. it's quite uncommon and usually fetches $30+, so good luck on a deal on that.
As far as Jedi Academy is concerned, $19.99 at Gamestop.com is probably your best bet. It's becoming increasingly harder to find. If you have XBox Live, it's easily worth the $20. Deals for those games are going to be hard since interest increased ten fold when Episode 3 came out.
I found a Jedi Outcast preowned for $14.95 at GameSquareOnline. Definately the cheapest you could find it.
The only Outcast I have is the one I got new/sealed at Circuit City in that $5 sale a long time ago for $4.99. Funny, no one else could get it to ring up for $4.99.